Most of radioactive nuclear species in the contaminated water stored in Fukushima Daiichi Nuclear Power Plant of Tokyo Electric Power Company are removed by ALPS treatment and coprecipitation with iron compounds. But it is only tritium, which is present as tritiated water (HTO), for the radioactive nuclear species to remain in the contaminated water at a higher than the regulated concentration when the contaminated water would be released into public water area.
The tritium concentration in the contaminated water is 0.6 to 5×106 Bq/L and the volume of the contaminated water is increasing by 400 m3/day every day. Thus, there is a need for development of a tritium-removing technology that can reduce the tritium concentration in the contaminated water at least to the environmentally allowable release concentration of 6×104 Bq/L or less (tritium concentration in sea water is 1 to 3 Bq/L) and has a processing rate of more than 400 m3/day.
Because the specific radioactivity of tritium (T) is 3.59×1014Bq/g, the concentration of tritiated water in the contaminated water is extremely low at 1.11 to 9.29×10−8 g/L, but it is desired for approximately 99% or more of tritiated water to be removed.
An idea of separating heavy water from light water, utilizing the difference in the crystallization temperatures of the gas hydrates between heavy water and light water is already known (Patent Document 1). Even though tritiated water would be used instead of heavy water, however, the concentration of tritiated water is extremely low in the contaminated water as described above, and when the gas hydrate containing tritiated water but not containing light water, is desired to be crystallized, the concentration is too low to form critical nucleus even though its precursor may be formed, and therefore, it is thus impossible in practice to crystallize the tritiated water.
Although there are many proposals separating the gas hydrate from liquid phase by floating or sedimentation separation, utilizing the difference in their specific densities, for separation of liquid phase and gas hydrate crystal (Non-Patent Literatures 1 and 2), in the case of the separation of tritiated water from light water, it is not possible to separate them sufficiently only by gravity, because the difference in their specific densities is very small, although the separation efficiency may depend on the gas used and the type of the hydrate structure formed.
Although a centrifugal method may be used, it is not practical as it demands high-speed and long-term operation for the separation because of the small particle diameter of the gas hydrate crystal.
Under the circumstances above, there is currently no industrially feasible method of separating tritiated water from light water.